In the construction of airplanes and certain vehicles the use of composite materials having separate and distinct layers of different materials poses certain problems to drilling these materials for assembly of the finished product. In drilling laminated layers of composite materials the layers of non-metallic material must be drilled at a higher cutter rotation speed with lower cutter feed advance than do the metallic layers of these materials.
Typical prior art positive feed drills are constructed to have a fixed cutter rotation speed and cutter feed rate for a particular setup to drill a particular hole or series of holes. These prior art devices typically would require dismantling, replacement of speed adjusting gears and reassembly of the particular tools in order to change the cutter rotation speed or feed rate. Therefore, using this type of positive feed drills for proper drilling of laminated workpieces would require either changing the drills upon penetration of the particular layers involved for them to be drilled at an optimum or appropriate cutter rotation speed and feed rate. In a mass production environment the changing of the drills several times to produce one hole is an unacceptable solution. Also, movement of drills from one workpiece to another may provide an alternate solution in some circumstances. However, for holes that must be drilled within precise tolerances the inaccuracies involved in moving the drills from one workpiece to another can present serious alignment difficulties as well as being extremely time consuming and burdensome in a manufacturing operation.
The power drill and control of this invention is constructed to solve the drilling problems presented above and provide a drill that can operate in various cutter speed and feed combinations with respect to the material through which it is drilling. The drill is equipped to sense the depth of the cutter in the material through which it is cutting as well as changing load conditions that effect the drilling operation.